This invention relates to ribbon-type conductors, and more particularly to a ribbon-type superconductor having low eddy current losses, and to a method for manufacturing such superconductor.
In superconducting magnets of the type used in high energy particle accelerators the use of high aspect ratio, solder filled, braided superconductor has been proposed. (See Physics Today, Apr. 1981, pg. 17). Superconducting wires are braided to form a ribbon-type conductor, substantially as described in U.S. Pat. No. 3,638,154 to Sampson, et al., date Jan. 25, 1972. The interstices of the braid are then filled with a solder, typically a nominal 97 weight % Sn, 3 weight % Ag solder, in order to provide stiffness and mechanical stability to the ribbon.
While the solder provided mechanical stability, it also provided a path for eddy currents which sometimes caused unexpectable losses in magnets made with such braided superconductor. This problem was particularly difficult to deal with, since eddy current losses varied from magnet to magnet.
Examination of samples of superconductors used in prototype magnets lead to the hypothesis that low eddy current losses were related to the formation of cracks in the solder. Based on this hypothesis, efforts were made to develop methods for producing controlled cracking of the solder. As a result of these efforts, two separate methods were developed. One method, conceived by T. Luhman and M. Suenaga, is the subject of a commonly assigned application Ser. No. 358,083, filed Mar. 15, 1982, now U.S. Pat. No. 4,431,862, and has as its object the production of a ribbon-type superconductor having a greatly increased interwire resistance so as to greatly reduce eddy current losses in the superconductor. This method, however, results in a superconductor which has a substantially reduced mechanical stability. Further, this method is only suitable for use with tin based solders.
A second method conceived by T. Luhman and C. Klamut is also the subject of a commonly assigned application Ser. No. 358,085, filed Mar. 15, 1982, now U.S. Pat. No. 4,426,550. This method produced a superconductor having substantial mechanical stability, but which had only limited improvement in eddy current losses.
Further problems with braided ribbon-type superconductors are a relatively poor "packing factor" (i.e., the amount of conductive material in a given volume of conductor), which limits the current carrying capability, and excessive deformation of individual strands when the braided superconductor is pressed into a ribbon.
Since no fully satisfactory solution to the problems of braided conductors had been found, it was decided to re-evaluate the original decision to use braid. This decision had been based on the known mechanical instability under tension of wound cables of high aspect ratio (i.e., ribbon-type cables having a high width to thickness ratio). Such cables tended to collapse to a tubular configuration under tension if they had an aspect ratio above about 12:1, and so could not conveniently be wound into magnet coils. Thus, solder filled ribbon-type conductors were proposed to provide a mechanically stable ribbon-type conductor. However, it was known from experience with wound conductors having a lower aspect ratio, so that mechanical stability under tension was not a problem, that such conductors had a higher packing factor, and thus high current carrying capability per unit volume. (Wound multifilar conductors are commonly known, and will hereinafter be referred to as Rutherford pattern conductors). Thus, it became an object of the subject invention to provide a high aspect ratio Rutherford pattern conductor which is mechanically stable under tension.
It is a further object of the subject invention to provide such a cable that also has satisfactory eddy current losses.